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How dangerous is my air sealing mistake?

ERIC WHETZEL | Posted in Green Building Techniques on

I just realized I made a mistake while air sealing around my new windows last year, and I’m wondering how serious it is.

I used the expanding Hannoband black foam tape for the tops and sides of all my windows, and then I also used Profil air-sealing tape on the interior and exterior to cover the Hannoband just for added insurance against air/water leaks.

My concern is with the bottom of my aluminum-clad wood windows.

From the inside, I put backer rod in the gap where the carpenters shimmed (1/16″ to about 1/4″, but mostly around 1/8” for all the windows). I then applied HF sealant over the backer rod from the interior. I also covered this with the Profil tape as well, just as added insurance against air intrusion.

Moving outside, I meant to stuff the remaining gap under the window with bits of Roxul Comfortboard 80 (Hammer and Hand talks about doing this in one of their videos), and then use the Profil to tape over the gap on the exterior between the window and the sill (European-style way of completely sealing all the way around the window, and recommended by my window rep).

Unfortunately, I realize now I forgot to add the Roxul. I can’t believe I forgot this detail.

In effect, because I only did the Profil tape outside I have about a 1/8″ thermal gap under all my windows, running about 2.75″ to the interior, between the Profil on the outside and the backer rod and HF sealant on the interior.

We had a low blower door score, and the smoke test around the windows showed no air leaks in this area, so I’m confident the backer rod, HF sealant, and Profil tapes worked, and this gap is at least protected from any direct air flow.

But is this thermal gap under the windows dangerous?

Do I need to worry about this gap not being completely insulated with the Roxul or a low-expansion spray foam?

Is condensation likely to form in this gap, especially on coldest/hottest days of the year?


And if condensation did form, would the Profil tape on the exterior side allow it to dry to the exterior before the moisture could harm the bottom of my window frames?

The HF sealant is vapor-closed, and the window buck sill is wrapped in Extoseal-Encors, a vapor-closed tape. If moisture develops in this gap it looks like it would either escape through the Profil tape to the exterior, or, worst case scenario, get wicked up into the bottom of my window frames.

On the exterior there’s a metal sill that covers the bottom front of the windows and the window buck sill out to the plane of the siding (the windows are placed mid-wall assembly), but hopefully the Profil tape would still have some drying potential behind these metal window sills?

I would hate to have to remove interior trim or exterior trim/metal sills  just to get to this small area under the windows to insulate it, but I also don’t want the bottom of my windows to get damaged by moisture.

I know this gap is a clear point of thermal bridging, but is it dangerous? Or is it like my locksets and door handles on my exterior doors — thermal bridging is clearly present, but small enough to safely ignore?

I’ll attach a crude diagram showing the details.

What, if anything, should I do?

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  1. GBA Editor
    Martin Holladay | | #1

    Condensation happens when warm interior air contacts a cold surface. That situation can't happen around your windows, because the interior tape seals off this gap from indoor air.

    What you have is dry outdoor air contacting a warm surface -- and that doesn't result in condensation.

    Could there be summertime condensation on days when your air conditioner is lowering the temperature of your air-sealing tape to 70 degrees? I doubt it. I wouldn't worry.

    1. ERIC WHETZEL | | #2

      Thanks Martin!

      I was starting to panic, thinking about having to remove finished surfaces, so thanks for putting my mind at ease.

      Thanks again!

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